Psychrometric : Dry, Moist, Saturated Air, Degree of saturation, Absolute, Relative Humidity, Dry and Wet Bulb Depression & Temperature [PDF]

What is Psychrometric?

Psychrometric is the branch of engineering science that deals with the study of moist air i.e. dry air mixed with water vapor or humidity. It also includes the study of the behavior of dry air and water vapor mixture under various set of conditions. Though the earth’s atmosphere is a mixture of gases including Nitrogen (N2), Oxygen (O2), Argon (Ar), and Carbon Dioxide (CO2), yet for  Psychrometry, It is considered to be a mixture of Dry air and water vapor only.

The Psychrometric terms are as follows:

Dry air

Pure dry air is a mixture of several gases such as Nitrogen, Oxygen, Carbon Dioxide, hydrogen, Argan, neon, helium, etc. The dry air is considered to have the composition as given in the following table:

The molecular mass of dry air is taken as 28.966 and the gas constant of air (R_a) is equal to (0.287 kJ/kg K) or (287 J/kg K). The molecular mass of water vapor is taken as 18.016 and the gas constant of air (Rv) is equal to (0.461 kJ/kg K) or (461 J/kg K).

Important point in dry air are as follows:

• Pure dry air does not ordinarily exist in nature because it always contains some water vapor.
• The term air wherever used in this text means dry air containing moisture in the vapor form.
• Both dry air and water vapor can be considered as perfect gases because both exist in the atmosphere at low pressure.

Thus all the perfect gas terms can be applied to them individually. The density of dry air is taken as 1.293 kg/m³ at pressure 1.0135 bar or 101.5 KN/m² and at the temperature (0 degree Celsius that is 273 Kelvin).

Moist air

Moist air is a mixture of dry air and water vapor. The amount of water vapor present in the air depends upon the absolute pressure and temperature of the mixture.

Saturated air

A mixture of dry air and water vapor in the air has diffused the maximum amount of water vapor into it is called saturated air. The water vapor usually occurs in the form of superheated steam as an invisible gas. However, When the saturated air is cooled the water vapor in the air starts condensing, the same may be visible in the form of moist, fog, or condensation on cold surfaces.

Degree of Saturation

The degree of saturation is the ratio of an actual mass of water vapor in a unit mass of dry air to a mass of water vapor in the same mass of dry air when it is saturated at the same temperature.

Humidity

Humidity is the mass of water present in 1 kg of dry air and is generally expressed in terms of a gram per kg of dry air ( g/kg of dry air). It is also called as specific humidity or humidity ratio.

Absolute Humidity

Absolute humidity is the mass of water vapor present in 1 m³ of dry air and is generally expressed in terms of a gram per cubic meter of dry air (g / m³ of dry air). It is also expressed in terms of grains per cubic meter of dry air. Mathematically, 1 kg of water vapor is equal to 15430 grains.

Relative Humidity

It is the ratio of an actual mass of water vapor in a given volume of moist air to the mass of water vapor in the same volume of saturated air at the same temperature and pressure. Relative humidity is briefly written as RH.

Dry Bulb temperature

It is the temperature of air recorded by a thermometer when it is not affected by the moisture present in the air. The dry bulb temperature is briefly written as DBT. This is denoted by t_d or t_db 

Wet Bulb Temperature

The temperature of air recorded by a thermometer when it’s bulb is surrounded by wet clothes exposed to the air is called as wet bulb temperature. Such a thermometer is called Wet bulb thermometer. The wet bulb temperature is briefly written as WBT. This is denoted by t_w or t_wb.

Wet Bulb Depression

This is the difference between the dry bulb temperature and wet bulb temperature at any point. The wet bulb depression indicates relative humidity of the air.

Dew Point Temperature

It is the temperature of air recorded by the thermometer when the moisture (Water vapor) present in it begins to condense. In other words, the Dew point temperature is the saturation temperature (Tsat_sat) corresponding to the partial pressure of water vapor p_v. This is usually denoted by TD_d. For saturated air, the dry, wet, and Dew point temperature is the same.

Dew point Depression

The difference between the Dry bulb temperature and the dew point temperature of the air is called as dew point depression.

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